I've been twice. Once just to Addis and the other time I went overland via buses from Somaliland.

In Addis I agree Yod Abyssinia is a good restaurant. Mostly locals when I went. It wasn't expensive by our standards but it's a nice place for locals.

Beyond the piazzas and churches you see when walking around in Addis if you wanted to go on a day trip, head to Mount Zuqualla. I stumbled upon some wild ceremony at the monastery at the top of the mountain when I hiked up. If you get a driver it can be done easily in a day. Otherwise I'm sure buses run but it would take some effort.

Harar is an interesting visit but it's a bit of a trip via bus. I wouldn't plan to spend your entire trip just in Addis.

With 5 days you could visit the north. There are a bunch of places there I'd like to see the next time I go.